This invention generally relates to solvent distillation units and more particularly to such units which during operation have power driven rotating scraper blades to prevent excessive accumulation of solid buildup on the heat exchange surface.
Extremely large quantities of organic solvents are used in producing many items of commerce. The plastics, paint and varnish, and printing industries and many others all use large quantities of organic solvents, both for producing the products themselves, to clean equipment, and for other uses. Much of this solvent becomes contaminated and must be disposed of or reprocessed.
In recent years there has become greater recognition of the hazard of solvent disposal which has resulted in increased regulation and hence cost. Moreover, the disposal of solvent without reclamation is a waste of valuable and limited resources as much of it has petroleum as its source.
The overall cost of a plant's solvent requirements, including the cost of disposal, is significantly reduced by the use of solvent distillation units which may be located and operated at the plant where the solvents are used. In accordance with such known distillation units, the unit may have a cylindrical tank with a conical bottom section. Within the tank are rotating scraper blades which scrape against the internal wall of the conical section as well as the lower portion of the side wall of the cylinder. Typically, the scraper blades are mounted on a vertical shaft that extends down the center of the tank and is rotated by a motor driven power unit externally at the top of the tank. A heat exchange surface surrounds the lower part of the tank including the conical section for purposes of heating the solvent within the tank. Means are provided for supplying contaminated solvent into the tank, and for the removal of solvent vapors to a condensor and reservoir for the reclaimed solvent. Also typically, the scraper blades of known units are segmented to accommodate for any unevenness in the surfaces over which they scrape as they rotate, and the blades themselves are of a relatively soft material, such as brass, compared to that of the tank which may be stainless steel. In addition to the distillation unit just described, other prior art of general interest is shown by U.S. Pat. Nos. 2,255,059, 3,428,106, 3,486,984, 3,522,016, and 4,095,307.
The purpose for the scraper blades of these known prior art distillation units is to prevent excessive buildup of solid waste materials on the internal walls of the vessel where the heat exchange is located and which would act as an insulator to substantially reduce the efficiency of the unit. Much of the used solvent material is so heavily contaminated that during the distillation process the solids quickly scale the heat exchange surfaces. Thus, the scrapers are operated continuously during the distillation process to prevent the buildup of scale and maintain the efficiency of the unit.
One of the problems with known units is in adjusting the scraper blades so that they maintain sufficient contact with the heat exchange surface to keep it free of excessive scale. The solid waste material in the solvent can be quite abrasive which causes the relatively soft material of the scraper blades to quickly wear. As the blades wear the build-up of scale on the heat exchange surface becomes greater until the efficiency of the unit is reduced to where blade adjustment is required. With known units this has been accomplished by adjusting the blades internally of the vessel. This means that a worker in the plant had to reach inside the vessel to adjust each one of the blades. Because these solvents are so toxic, the unit had to be cleaned before the blades could be adjusted. The task of cleaning the unit and adjusting the blades was very time-consuming and distasteful. Down times of two and three days were not uncommon.
Applicant has solved this problem in providing a solvent distillation unit where the scraper blades can be adjusted externally of the vessel, and without requiring the unit to be entirely shut down and cleaned. The adjustment can be made in a few minutes without requiring access into the inside of the vessel. More specifically, in accordance with the present invention the scraper blades rotate to scrape the internal surface of the conical bottom of the vessel. The scraper blades are mounted on a vertical shaft which is powered by a motor drive externally at the top of the vessel. Adjustment of the scraper blades outwardly against the conical surface is accomplished through vertical adjustment of the shaft externally at the top of the vessel where the motor drive is located. As a result, the present invention provides a distillation unit which maintains efficient heat exchange for economical reclamation of waste solvent materials and minimizes the expensive down time that had been required to adjust the scrapers. The invention further provides means for preventing overadjustment of the scraper and consequent damage to the heat exchange surface and hazard that exists by explosion should the harder metal of the blade holder scrape the vessel wall.
These and further objects and advantages of the invention are apparent from the drawings and detailed description to follow.